Apparatus for heating molten metals



Nov. 21, 1967 J. T. VAUGHAN ET AL 3,354,256

APPARATUS FOR HEATING MOLTEN METALS Filed Dec. 10, 1964 I N V ENTORS.

JOHN T. VAUGHAN 8 GERHARD W. SEULEN w ww,

ATTORNEYS United States Patent Ofifice 3,354,256 APPARATUS FOR HEATING MOLTEN METALS John T. Vaughan, Tampa, Fla., and Gerhard W. Seulen,

Remscheid-Hasten, Germany, assignors to Alco Standard Corporation, Philadelphia, Pa., a corporation of Ohio Filed Dec. 10, 1964, Ser. No. 417,395

' 8 Claims. (Cl. 13-23) ABSTRACT OF THE DISCLOSURE nected so as to carry an electric current passing in the opposite direction to that flowing in the charge thereby attenuating the inductive effect. A feature also is the construction of the electrode which permits continuous immersion thereof in the metal charge at a constant depth so as to minimize changes in current density due to possible fluctuations in the height of the metal charge.

The invention is particularly applicable to the superheating of a stream of molten cast iron as it flows from a cupola furnace to a forehearth or mixer and will be described with particular reference thereto although it will be appreciated that the invention has broader applications and may be utilized for heating any molten metal whenever it is desirable to increase its latent heat.

It is known that cast iron melted in a cupola furnace does not have the required casting temperature nor in most cases the required chemical composition for direct subsequent casting. For this reason, cast iron tapped directly from the cupola is generally collected in forehearths or mixers where alloying ingredients may be added and where the temperature of the melt may be increased as desired by supplementary heating.

Such supplementary heating heretofore has been accomplished in a number of different ways, primarily by inducing electric currents to flow in the molten metal or by the use of a gas heated muffle furnace, both of which have certain inherent difiiculties.

In accordance with the broadest aspect of the present invention, this supplementary heating is accomplished by a direct flow of electric current in the metal between immersed metallic electrodes connected to a source of electric current and when used with cast iron the electrodes are suitably cooled by means of a cooling medium flowed through the interior of the electrodes.

More particularly in accordance with the present invention, a pair of electrodes are inserted at spaced points along a flowing stream of metal and electric currents are circulated between the electrodes through such stream.

Where a continuous stream of molten metal is to be heated by such direct current resistance heating, certain problems do arise. One of these is the inductance of the electrical circuit which, because of the relative wide spacing of the electrodes, can become quite large. This in ductance arises not only from the power leads to the electrodes, but also from the length of the current path in the molten metal itself.

Another problem encountered in resistance heating a continuous molten metal stream is the magnetic field 3,354,256 Patented Nov. 21, 1967 existing around the current carrying elements of the circuit such as the power leads and the electrodes. These magnetic fields link with the magnetic field about the molten metal stream producing forces, which occurring axially, may cause a physical separation of the molten metal thus giving rise to an arcing condition.

Thus, further in accordance with the invention, this inductance and the interaction of the magnetic fields is reduced by running the exterior power leads as close as possible to each other and/or to the exterior surface of the metal in which the current is flowing.

In accordance with this aspect of the invention, the inductance is reduced and a completely magnetic field-free metal path is provided by surrounding the path of the flowing molten metal between the electrodes with a sleeve of electrically conductive material and connecting the sleeve, the electrodes and the metal between the electrodes in electrical series relationship with an electrical power source such that the electrical currents in the molten metal and in the sleeve are at any one instant flowing in opposite directions. With such an arrangement, the inductance can be reduced to zero resulting in a near unity power factor and any objectionable electromagnetic forces are practically eliminated.

Another problem with such direct electrical resistance heating is in maintaining the flow of current constant while the level of the molten metal varies, thus changing the depth of immersion of the electrodes in the metal. Such changes in the depth of immersion change the area of contact of the electrodes with the molten metal and thus the resistance and current in the circuit.

In accordance with another aspect of the invention, the electrodes have a coating of electrically insulating material extending from a point spaced from the lower end of the electrode but below the lowest level of immersion to a point at least above the maximum point of immersion of the electrode.

Still another problem with such direct electrical resistance heating is the type of electrode used, particularly with the higher melting temperature metals such as molten cast iron. For example, if an electrode is employed having a melting temperature higher than the melting temperature of the cast iron, such as carbon, then the problem of contamination of the molten metal arises. Tantalum or other high melting temperature electrodes are prohibitively expensive and have a high electrical resistance. On the other hand, if electrodes are employed having a melting temperature less than that of the molten metal such as copper, artificial cooling must be employed. If Water is employed for such artificial cooling, then there is always present the danger of explosion if, for example, the molten metal should come in contact with the water for any reason such as a fracture of the copper electrode or in the event the cooling water supply should fail and the copper electrode melts.

Thus, and in accordance with another aspect of the invention, an electrode having a conventional cooling passage therein is provided in combination with a source of a molten metal having a melting point substantial-1y below the melting temperature of the electrode metal and of the metal stream being superheated which source circulates the molten metal continuously through the passages of the electrode to cool it.

In an especially advantageous form of the invention, the electrode is provided with a protective metal jacket completely encasing the portion of the electrode exposed to the molten metal stream and such jacket is formed of a metal having a substantially higher melting point than that of the electrode metal and is relatively unaffected by physical contact with the metal being heated so as to avoid the problem of contamination and increase the cooling efificiency of the electrode.

The principal object of the present invention is the provision of a new and improved arrangement for adding supplementary heat to molten metal as it is being transferred from one container to another.

Still another object of the invention is the provision of a new and improved apparatus for the direct resistance heating of a continuously flowingmetal stream in which the electric circuit associated with the apparatus will have a minimum inductance.

Another object of the invention is the provision of a new and improved apparatus of the general type referred to wherein the metal flowing in the stream will be relatively free of any electromagnetic forces.

Still another object of the invention is the provision of a new and improved electrode adapted to be immersed in molten metal and to conduct electric current thereto, which electrode is so arranged that the resistance between the electrode and the molten metal will not vary as the depth of immersion of the electrode in the molten metal is varied.

Still another object of the invention is the provision of a new and improved arrangement for the cooling of electrodes adapted to be immersed in molten metals,

wherein the danger of an explosion due to the molten metal coming in contact with cooling water is entirely eliminated.

Yet a further object of the invention is to provide an electrode as referred to above which is protected from the metal being heated to both increase the cooling efficiency for the electrode and to avoid contamination in the molten metal.

These and other objects and features of the invention will become apparent from a consideration of the following description which proceeds with reference to the accompanying drawings wherein:

FIGURE 1 is a longitudinal cross-sectional view of a preferred embodiment of the invention; and

FIGURE 2 is an enlarged partial cross-sectional view of one electrode constructed in accordance with the invention taken along line 22 of FIGURE 1.

Referring now to the drawings wherein the showings are for the purpose of illustrating a preferred embodiment of the invention only and not for the purpose of limiting same, FIGURE 1 shows, in more or less schematic form, an apparatus including a refractory member 12 and a pair of spaced electrodes 14, 16 which are adapted to be electrically connected to a source of electric current generally indicated by the numeral 18. It should the understood that-since the invention deals with resistance heating of molten metals as opposed to induction heating, for example, it makes no difference in the actual heating of the metal whether an alternating or direct current is used. However, the description hereinafter will be with reference to an alternating current power source 18.

The refractory member 12 has longitudinally spaced ends 20, 22 which define inlet and outlet openings 24, 26 of a substantially straight passageway 28 extending through the member 12. The passageway 28 has a known length and cross-sectional area, the dimensions of which are predetermined for reasons to be explained. For purposes of description, it is assumed that an endless supply of molten metal which is to be superheated is being discharged from a suitable vessel, such as a cupola furnace, into the inlet 24 of member 12. The end 22 of the member 12 is lower than the end 20 so that the passageway 28 is inclined slightly to allow a gravity flow of the metal and the cross-sectional area and inclination of passageway 28 are such that a continuous metal stream 32 is established which completely fills the passageway 28 from end to end. Superheated metal is discharged from apparatus 10 into an appropriate container such as a transfer ladle positioned below a pouring lip 38 at a rate approximately equal to the rate molten metal is being added at the inlet opening 24. It will be appreciated that the form of refractory member 12 may vary and while a straight passageway 28 is shown, it would not be a departure from the invention to provide a curved passageway, for example, in which the member 12 was in the form of a helix. For purposes of illustration, with the apparatus10 being charged with molten cast iron at a rate of 20 tons per hour, a rate of flow of approximately 19 feet per minute is achieved if passageway 28 has a diameter of approximately four inches and is about 13 feet long.

Electrodes 14, 16 are suitably suspended over the inlet and outlet openings 24, 26 respectively and extend downwardly so that their tips 42 are immersed in the continuous metal stream 32 flowing through passageway 28. The metal stream 32 is heated by the current flowing between the electrode tips 42 as will be more fully discussed hereinafter. Since the metal is in a molten condition, its resistivity is not very great which necessitates a Wide spacing of electrodes 14, 16 in order to obtain any significant resistance heating of the metal. For example, with an electrode spacing of approximately thirteen feet, molten cast iron with a cross section as above indicated has a resistance of only about 1.6 milliohms.

Electrodes 14, 16 being identical, a description with respect to one will apply equally to the other and the same numerals will be used to identify like parts of each. With reference to electrode 16, a coolant fitting 44 is provided at the top which couples the electrode to a source of cooling fiuid and pump, not shown, by means of liquid delivery and return lines 45, 46. Referring now to FIGURE 2, the lower portion of electrode 16 is shown in cross section and includes an elongated cylindrical core 48 which extends the full length of the electrode. The core 48 is formed of a high conductivity metal, such as copper, and has an internal passage 49 into which extends a pipe 50 which is suitably connected at the upper end to fitting 44 in any well known manner so as to receive coolant from delivery line 4-5. The cooling liquid is introduced by the pipe 50 at the tip 42 and is recirculated to the coolant pump via passage 49 and return line 46 as is conventional. In ac cordance with the invention the cooling liquid is prefer= ably molten metal. Any suitable low melting metal alloy such as those of lead, tin, or bismuth may be used. The

important point is that the melting point of the coolant should be substantially below that of the core 48 and the coolant should be some liquid other than water for reasons made clear hereinafter.

In accoradnce with a further aspect of the invention, the core 48 is completely encased with a jacket 52 formed of a metal having a melting point substantially above that of metal core 48. In addition, the jacket 52 is made of a metal which is physically unaffected by constant exposure to the molten metal 32, and while possibly such metal is not considered a good electrical conductor, it is preferably a sufiicient conductor to avoid high power losses. For example, in superheating cast iron, the jacket 52 may also be of cast iron or some other ferrous base metal. Actually, the jacket 52 need only encase the tip 42 of the electrodes 14, 16 as will be seen from the following and terminals 54 may engage the core 48 directly. Inasmuch as jacket 52 is a more refractory metal than is commonly classed as a good conductor, some power loss is to be expected in transfer of current from the high conductivity core 48 to the molten metal 32 which is being heated. These losses are not substantial, however, when compared with those resulting from cooling of the electrodes and may be disregarded.

Referring to FIGURE 1 again, it may be seen that electrode 14 is immersed in the molten metal 32 to a greater extent than is electrode 16. This is due to the arrangement of the refractory member '12 and the positioning of pouring lip 38. However it should be understood that whatever causes fluctuations or differences in metal height between the electrodes, it is importantfor eflicient electrical'resistance heating to maintain a constant depth of immersion of the tips 42 of electrodes 14, 16. To insure that any change in the height of metal does not vary the area of electrical contact between the electrode tips 42 and the molten metal, an electrically insulative sleeve 55 is provided.

In accordance with the invention, the sleeve 55 completely encircles the jacket 52 and extends from a point spaced at given distance above the lower end of the electrode 16 to a point below terminal 54. Thus, the area of tip 42 through which current passes remains constant regardless of variations in metal height. The lower end of sleeve 55 is below the lowest level of immersion While the upper end is above the highest level of immersion. For purposes of illustration, it may be assumed that the height of metal 32 in the inlet opening 24 is at the highest level of immersion which will exist during operation of the apparatus while the height of metal 32 in the outlet opening 26 is at the lowest level of immersion. In each case, however, the area of tip 42 exposed to molten metal remains the same.

In the invention, the metal stream 32 flowing between electrodes 14, 16 is actually a part of the resistance heating circuit 40. In the preferred embodiment shown in FIGURE 1, the circuit 40 also includes a sleeve member 58 extending in coaxial relationship with the passage 28 completely around member 12 and terminating near ends 20, 22 as close as possible to electrodes 14, 16. The sleeve member 58 is formed of a high conductivity metal, such as copper, and is a conductor of electric current between the power source 18 and electrode 16. A connection at one end of sleeve member 58 is made to a power line 59 leading to the power source 18 and at the opposite end to jumper cable 60 which connects with terminal 54 of electrode 16. The power circuit 40 is completed by the connection of power line 62 from the power source 18 directly to the terminal 54 of electrode 14. Thus, in spite of the relatively wide spacing of electrodes 14, 16, the, invention permits power cables 59, 62 to extend from the power source 18 to the apparatus 10 in closely spaced, side by side relationship. More-over, the arrangement of conductive sleeve member 58 coaxially with the metal stream 32 which is being heated by current flowing in it, permits these current carrying elements of the circuit 40 to be spaced as closely together as the refractory walls of member 12 will permit with safety. These are important features of the invention as will be explained.

The particular advantages of the invention are more fully understood by considering the following explanation. In operation, once the metal stream 32 is established in passage 28, power is turned on and the stream 32 flowing between electrodes 14, 16 is heated directly by the electric current. Aside from the normal resistive losses of the power circuit 40 in delivering current to the electrodes 14, 16 and in transferring it to the metal stream 32, inductive losses of a more substantial nature are virtually unavoidable in such a circuit when it is considered that the metal stream 32 flowing between electrodes 14, 16 is also part of the circuit. The molten metal 32 represents a conductor of considerable length due to the necessary Wide spacing of electrodes 14, 16 in order to obtain any significant resistance heating and as a result contributes substantially to the inductance of the circuit 4t}. Also, the inductance from the power lines 59, 60 will be considerable, and adding the two, the total inductive loss in the circuit will be so great that the installation of capacitors will be required in order to avoid greatly reducing the power factor.

However, in the present invention, these inductive losses are kept low or eliminated. The conductive sleeve member 58 plays a double role in this regard. That is, though the electrodes 14, 16 are spaced widely apart, the power cables 59, 62 may extend in closely spaced, side by side relationship so as to limit their inductive contribution to the circuit 40. This close spacing of the power cables 59, 62 is an aspect of the invention arising from the fact that the sleeve member 58 acts as a conductor between electrodes 14, 16, thus in effect, achieving the same result as though the electrodes 14, 16 were spaced side by side.

The sleeve member 58 also limits the inductance in another way. Since both it and the quantity of metal flowing between electrodes 14, 16 are conductors in the circuit 49, they will contribute to the total inductance. However, since the sleeve member 58 extends in closely spaced, coaxial relationship with the metal stream 32, their inductance will mutually cancel. That is, at any one instant, the current flowing in the sleeve member 58 is opposite to that flowing in the metal stream 32 and as a result of the close spacing, the respective magnetic fields around each conductor will be destroyed. Thus, with the invention, the total inductive loss is minimized and the power factor is maintained near unity without the need of any capacitance in the circuit.

Moreover, the closely spaced relationship of the metal stream 32 and the conductive sleeve member 58 has another important function particularly applicable to heating molten metals by direct resistance heating. That is, due to the large currents used in heating the metal, strong magnetic fields will be produced which will link with portions of the metal stream 32 being heated, thus producing electromagnetic forces, which if occurring in an axial direction, may cause a physical separation of the metal stream 32 flowing between the electrodes causing an arcing condition. In the present invention, the coaxial arrangement of the sleeve member 58 and metal stream 32 insures a completely field-free space through which the metal flows.

Aside from these factors, an additional problem in connection with direct resistance heating of a continuous metal stream arises from the fact that the electrodes are immersed directly in a moving stream of high temperature metal and are subjected to its heat as well as to physical attack.

Also, if an electrode is employed having a melting temperature less than that of the metal being heated, then cooling becomes necessary. Cooling with water is obviously impractical in view of the danger of a rupture occurring in the electrode permitting the cooling water to come into contact with the molten metal and possibly causing an explosion.

Moreover, since the degree of heating is controlled by the amount of current flowing between the electrodes, it is important for power control that the depth of immersion of the electrodes be kept constant. This is a particular problem where in an apparatus of the type described, the level of molten metal adjacent the electrodes may vary between minimum and maximum points of immersion in accordance with the rate of feed.

Thus, as previously described, electrodes 14, 16 are cooled with liquid metal when immersed in a metal having a much higher melting point than they do and are provided with a high conductivity core 48 to carry the heavy electric current down to the tip 42, but are protected at least over the part immersed in the molten metal, by a metal jacket 52. The jacket 52 has a much higher melting point than that of the core 48 and is relatively unaffected by constant physical contact with the metal 32 being heated. For example, if the metal being superheated is cast iron and the core 48 is of copper, cooling is required and the jacket 52 is preferably a ferrous base metal such as cast iron. Depending upon the temperature of the coolant inside, the existing temperature of the molten metal to be heated on the outside, and other factors such as the thickness of the cast iron sleeve, the outer surface temperature of the electrode tip 42 may be as much as five times greater than the temperature on the inside of the core 48 due to the difference in thermal conductivity between copper and cast iron and the cooling on the inside due to the circulating liquid metal.

Another feature of importance with respect to the inventive electrodes 14, 16 is the outer refractory sleeve 55 which insures that a constant tip area is immersed in the molten metal 32 regardless of the height of molten metal at the electrodes. The sleeve 55 is made of electrically insulative material which also is a refractory with respect to the metal 32, for example, a molded asbestos sleeve may be used of whatever thickness necessary.

It should be appreciated that the invention has been described in connection with a single embodiment for the purpose of distinctly describing a preferred form of the invention, however, it is clear that modifications may be proposed and certain obvious changes made without departing from the intended spirit and scope of the present invention as described by the appended claims.

Having thus described our invention, we claim:

1. A resistance heating apparatus for heating a continuous stream of molten metal comprising a refractory member defining a passageway which said molten metal flows,

spaced electrodes extending into said passageway and immersed in said metal,

a source of electric current adapted to be connected in electrical series relationship with said electrodes and metal being heated,

a conductive sleeve member arranged in coaxially, closely spaced relationship to said passageway co-extensive therewith to points adjacent each electrode, and

electrical conductor means for connecting said power source, electrodes, sleeve member and metal being heated in series relationship whereby currents flowing in the sleeve member and metal being heated are flowing in opposite directions at any given instant.

2. Apparatus according to claim 1 wherein said electrical conduct-or means include a first power cable connected between one of said electrodes and the power source and a second power cable extending in closely spaced, side-by-side relationship to said first power cable connected between said power source and the end of said conductive sleeve member closest to' said one electrode.

3. A liquid cooled electrode adapted for direct resistance he-ating of molten metal-s comprising:

' an electrically conductive member having a hollow metal tip portion adapted to be immersed in the molten metal charge to be heated but which has a melting point substantially below that of the metal being heated,

a liquid cool-ant having a melting point substantially below that of the tip portion circulated to the tip portion to prevent it from melting, and

a metal jacket completely encasing and in electrical contact with said tip portion and being formed of a metal of similar properties to that being heated to avoid contamination of the charge.

4. The electrode according to claim 3 including in addition;

an electrically insulative sleeve surrounding said electrically conductive member and in contact therewith above said tip portion at least from a point below the through lowest level of immersion of said tip portion to a point above the highest level of immersion in said charge where the level of said charge is subject to fluctuations while the electrode is fixed.

5.- A resistance heating apparatus comprising a tubular refractory vessel metal to be heated,

a pair of spaced electrodes each immersed within the molten metal adjacent opposite ends of said tubular Vessel adapted to introduce an electrical current into the charge for heating it,

an electrically conductive sleeve closely surrounding the metal charge and substantially coextensive longitudinally thereof, and

means for passing an electrical current through said sleeve in a direction opposite to the current passing through said charge.

6. A resistance heating apparatus according to claim 5 wherein said sleeve is electrically connected at one end to one of said electrodes whereby it is in electrical series relationshrip with said electrodes and metal charge.

7. A resistance heating apparatus according to claim 5 wherein the tubular vessel is generally horizontally disposed and includes inlet and outlet openings at each end extending upwardly from the space containing the charge, said molten metal charge entering through said inlet opening and leaving through said outlet opening at substantially equal rates of flow while maintaining the tubular vessel continuously filled with molten metal.

8. A resistance heating apparatus according to claim 7 wherein one electrodeof said pair of electrodes is immersed in the molten metal at the inlet opening and the other at the outlet opening.

for molten metals into which is charged molten References Cited UNITED STATES PATENTS 1,050,189 1/1913 Westley 13-23 1,056,456 3/1913 Schemmann et al. i 13-23 1,628,376 5/1927 Valentine 13-23 1,874,417 8/1932 Baily 13-23 2,419,383 4/1947 Ames 13-23 2,599,179 6/1952 Hopkins 13-23 X 2,697,130 12/1954 Korbelak 13-23 X 2,908,738 10/1959 Rough 13-23 X 2,933,545 4/1960 Koefer 13-17 3,036,011 5/1962 Miller -106 X 3,089,840 5/1963 Carter et al. 165-106 X FOREIGN PATENTS 198,952 2/ 1907 Germany.

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RICHARD M. WOOD, Primary Examiner. V. Y. MAYEWSKY, Assistant Examiner. 

5. A RESISTANCE HEATING APPARATUS FOR MOLTEN METALS COMPRISING A TUBULAR REFRACTORY VESSEL INTO WHICH IS CHARGED MOLTEN METAL TO BE HEATED, A PAIR OF SPACED ELECTRODES EACH IMMERSED WITHIN THE MOLTEN METAL ADJACENT OPPOSITE ENDS OF SAID TUBULAR VESSEL ADAPTED TO INTRODUCE AN ELECTRICAL CURRENT INTO THE CHARGE FOR HEATING IT, AN ELECTRICALLY CONDUCTIVE SLEEVE CLOSELY SURROUNDING THE METAL CHARGE AND SUBSTANTIALLY COEXTENSIVE LONGITUDINALLY THEREOF, AND MEANS FOR PASSING AN ELECTRICAL CURRENT THROUGH SAID SLEEVE IN A DIRECTION OPPOSITE TO THE CURRENT PASSING THROUGH SAID CHARGE. 